Differential oscillator

ABSTRACT

A differential oscillator based on a first Colpitts oscillator and a mirror image Colpitts oscillator that is coupled to the first Colpitts oscillator. This differential oscillator outputs differential voltage signals that are about 180 degrees out of phase. The differential oscillator may also be adapted to form a voltage controlled oscillator (VCO) such that the differential voltage signals output by the VCO can be varied. A transceiver for telecommunication devices such as cellular phones may use differential oscillators to generate a carrier signal on which a voice or data signal is modulated and the same differential oscillators to assist isolation of the voice or data signal from received signals.

FIELD OF THE INVENTION

The field of the present invention relates generally to differentialoscillators and more specifically, to differential dual Colpittsoscillators.

BACKGROUND OF THE INVENTION

An oscillator generates a periodic signal. Accordingly, an oscillatormust have a self-sustaining mechanism that allows its own noise to growand eventually become a periodic signal. Oscillators having a periodicsignal whose frequency falls in the radio frequency range (RF) are oftenreferred to as RF oscillators.

Many RF oscillators use feedback circuits to generate the periodicsignal. In these RF oscillators, a frequency-selective network such asan inductor-capacitor (LC) tank is included in the feedback loop inorder to stabilize the frequency. The frequency-selective network isalso called a “resonator.” The nominal frequency of oscillation is oftendetermined by the characteristics of the circuit including, for example,the resonance frequency of the LC tank.

Most discrete RF oscillators incorporate only one active device (e.g., atransistor). There are two reasons for using a one-transistor topology:noise is minimized and costs are reduced.

FIG. 1 illustrates common collector configuration of a traditionalColpitts oscillator that is well known in the art. This Colpittsoscillator has only one transistor, a bipolar junction transistor 12.The transistor 12 has its collector connected to a voltage sourceV_(cc). The base of the transistor 12 is connected to an inductor 14 vianode 16. The inductor 14 has an internal resistance, shownrepresentatively by resistor 18. Resistor 18 is not a resistanceseparate from the internal resistance of the inductor 14. A resistor 20is connected between the voltage source V_(cc)and the node 16. Node 16is also connected to one side of a capacitor 22 and the other side ofthe capacitor is connected to the emitter of the transistor 12 throughnodes 24 and 26. A capacitor 28 is connected between node 24 and ground.A resistor 30 is connected between node 26 and ground.

Such standard Colpitts oscillators are well known and theircharacteristics have been well studied. Colpitts oscillators behave in apredictable fashion and are easy to implement. Nodes 24 and 26 arephysically the same node and carry the output voltage.

One advantage of this Colpitts oscillator is that it has a low outputimpedance and therefore is less influenced by the circuits which followit. However, the output signal on node 26 is a single signal. Hence, ifthe circuit designer requires differential signals, this Colpittsoscillator cannot output such signals. Therefore, there is a need tohave an oscillator circuit that can output differential voltage signalswhich are accurate and have good harmonic content. A signal having goodharmonic content is one that has a primary resonant frequency and whosehigher order harmonic frequencies are suppressed. It is desirable tohave a “balanced” signal, that is, one whose two components are 180degrees out of phase. Differential voltage signals that are notprecisely out of phase result in reduced signal amplitude or phaseerrors which may degrade the quality of systems that use oscillators.For example, telecommunication and cellular telephone systems that usenoisy or inaccurate oscillators may suffer from perceptibly degradedvoice qualities.

The phase noise of an oscillator based on a LC tank usually depends onthe Q of the tank. The higher the Q of the LC tank, the sharper theresonance and the lower the phase noise skirts. The Q represents howmuch energy is lost as the energy is transferred from the capacitor tothe inductor and vice versa. The Q, phase noise and other attributes ofColpitts oscillators have been well studied and are well known to thoseof skill in the art.

Oscillators may be used to form other devices including voltagecontrolled oscillators (VCOs). On a larger scale, oscillators may beused in wireless communication systems such as mobile radiocommunication systems and cellular telephone systems. Hence,improvements in oscillators lead to improvements in other systems.

SUMMARY OF THE INVENTION

In accordance with the purpose of the invention as broadly describedtherein, there is provided a differential oscillator.

In particular, a first embodiment of a differential oscillator has afirst Colpitts oscillator that is coupled to a mirror image Colpittsoscillator through a coupling network comprising two inductors, a firstresistor that is connected between the base of the transistor of thefirst Colpitts oscillator and the voltage source, and a second resistorthat is connected between the base of the transistor of the secondColpitts oscillator and the voltage source.

A second embodiment of the differential oscillator comprises a firstColpitts oscillator that is coupled to a mirror image Colpittsoscillator through a coupling network comprising a cross-coupledtransformer having two inductors, two DC blocking capacitors connectedto the base of the transistors of the Colpitts oscillators, a firstresistor that is connected between the base of the transistor of thefirst Colpitts oscillator and the voltage source, and a second resistorthat is connected between the base of the transistor of the secondColpitts oscillator and the voltage source.

A third embodiment of the differential oscillator comprises a firstColpitts oscillator that is coupled to a mirror image Colpittsoscillator through a coupling network comprising a single inductor and aresistor that is connected between the voltage source and the midpointof the inductor.

A fourth embodiment of the differential oscillator comprises a firstColpitts oscillator that is coupled to a mirror image Colpittsoscillator through a coupling network comprising two substantiallyidentical inductors and a resistor that is connected between the voltagesource and a node between the two inductors.

The differential oscillator may be used to form a VCO. In particular, afirst embodiment of such a VCO comprises a first Colpitts oscillator, amirror image Colpitts oscillator that is coupled to the first Colpittsoscillator through an inductor, and a varactor or variable capacitorcoupled across the inductor. A resistor may be connected between themidpoint of the inductor and a voltage source in order to bias thetransistors.

A second embodiment of a VCO comprises a first Colpitts oscillator, amirror image Colpitts oscillator that is coupled to the first Colpittsoscillator through two identical inductors, and two varactors orvariable capacitors coupled in series across the inductors. This secondembodiment further includes DC blocking capacitors coupled to thevaractors and a voltage control signal to control the varactors.

The differential oscillator may be used in any kind of system. Forexample, it can be used in a multi-band transceiver for transmitting andreceiving RF signals in one of a plurality of frequency bands. Oneembodiment of a transceiver that uses the differential oscillatorcomprises a transmit circuit which modulates a voice or data signal ontoa carrier signal. The carrier signal is a differential very highfrequency (VHF) signal outputted from a differential dual Colpittsoscillator, such as the fourth embodiment described above. Thismodulated carrier signal is filtered and processed and transmitted as aradio frequency signal. The transceiver includes a receiver circuit thatreceives radio frequency signals and amplifies and filters them. Thereceiver circuit isolates the voice or data signal from the carriersignal by using filters and mixers. To do so, the receiver circuit usesa differential ultra high frequency (UHF) signal generated by adifferential dual Colpitts oscillator, such as the fourth embodimentdescribed above. By using differential voltage signals, the transceiveris more immune to external noise and interference because differentialsignals tend to cancel the effects of external noise and interference onthe transceiver.

A first, separate aspect of the differential oscillator invention is itsability to generate differential voltage signals.

A second, separate aspect of the differential oscillator invention isits ability to generate differential voltage signals that are preciselyout of phase by 180 degrees.

A third, separate aspect of the differential oscillator invention is itsability to reject common mode oscillation.

A fourth, separate aspect of the differential oscillator invention isits ability to generate differential voltage signals that are preciselyout of phase by 180 degrees while rejecting common mode oscillation.

A fifth, separate aspect of the differential oscillator invention isthat it generates differential voltage signals by using only two activedevices (e.g., two transistors).

A sixth, separate aspect of the differential oscillator invention is itsuse of two mirror image Colpitts oscillators.

A seventh, separate aspect of the differential oscillator invention isits use of a first Colpitts oscillator coupled to a mirror imageColpitts oscillator by a low impedance element.

An eighth, separate aspect of the differential oscillator invention isits use of a first Colpitts oscillator coupled to a mirror imageColpitts oscillator by a low impedance element where the two Colpittsoscillators share an inductive element.

A ninth, separate aspect of the differential oscillator invention is itsuse of a first Colpitts oscillator coupled to a mirror image Colpittsoscillator by a low impedance element where the two Colpitts oscillatorseach have an inductive element.

A tenth, separate aspect of the invention is a voltage controlledoscillator which generates differential voltage signals accurately.

An eleventh, separate aspect of the invention is a voltage controlledoscillator which generates differential voltage signals that areprecisely out of phase and which rejects common mode oscillation.

A twelfth, separate aspect of the invention is a transceiver which usesthe differential dual Colpitts oscillator to generate a carrier signalthat is more immune from external noise.

A thirteenth, separate aspect of the invention is a transceiver thatuses differential dual Colpitts oscillators to generate a carrier signalonto which a voice or data signal is modulated and the same differentialdual Colpitts oscillators to assist in isolating the voice or datasignal from a received modulated signal.

A fourteenth, seperate aspect of the invention is any of the foregoingaspects, singly or in combination.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a traditional Colpitts oscillator known in theprior art.

FIG. 2 is a schematic of a first embodiment of a differentialoscillator.

FIG. 3 is a graph of the time response of the differential oscillator ofFIG. 2.

FIG. 4 is a schematic of a second embodiment of a differentialoscillator.

FIG. 5 is a schematic of a third embodiment of a differentialoscillator.

FIG. 6 is a schematic of a fourth embodiment of a differentialoscillator.

FIG. 7 is a schematic of a first embodiment of a voltage controlledoscillator that uses the differential dual Colpitts oscillator of FIG.5.

FIG. 8 is a schematic of a second embodiment of a voltage controlledoscillator that uses the differential oscillator of FIG. 6.

FIG. 9 is a block diagram of a transceiver that uses the differentialoscillator.

FIG. 10 is a graph of the time response of the differential oscillatorof FIG. 6 using the same scale as FIG. 3.

FIG. 11 is a graph of the time response of the differential oscillatorof FIG. 6 using a first scale of time which illustrates that the twosignals are 180 degrees out of phase.

FIG. 12 is a graph of the time response of the differential oscillatorof FIG. 6 using a second scale of time which illustrates that the twosignals are 180 degrees out of phase.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 illustrates one embodiment of the differential oscillatorinvention. Specifically, it is a differential dual Colpitts oscillatorcomprising first and second Colpitts oscillators which are generallycorrelated with one another, and which are coupled together through acoupling network. In one example, the differential dual Colpittsoscillator 40 of FIG. 2 comprises a traditional Colpitts oscillator 42which is coupled to a mirror image Colpitts oscillator 44, that is, anoscillator 44 which is generally correlated with oscillator 42. Thedotted line A—A in FIG. 2 illustrates the plane of symmetry between thetwo Colpitts oscillators and depicts how they are mirror images of eachother. In one example, each Colpitts oscillator 42, 44 is the same asthe Colpitts oscillator previously shown in FIG. 1.

Specifically, in one example the first Colpitts oscillator 42 comprisesa single transistor 52 which may be, for example, a MRF947bipolarjunction transistor manufactured by Motorola Corporation. The base ofthe transistor is connected to one end of an inductor 54 through node56. Resistor 58 represents the internal resistance of the inductor 54,not a separate resistor. The base of the transistor 52 is also connectedvia node 56 to a resistor 60. The other end of the resistor 60 isconnected to a voltage source V_(cc). A capacitor 62 is connected to thebase of the transistor through node 56. The other side of the capacitor62 is connected to the emitter of the transistor 52 through node 64which is the same as node 66. A capacitor 68 is connected between node64 and ground while a resistor 70 is connected between node 66 andground.

In one example, the mirror image Colpitts oscillator 44 likewisecomprises a transistor 82 that is substantially identical to thetransistor 52. The base of the transistor 82 is connected through node86 to an inductor 84. The resistor 88 is the internal resistance of theinductor 84, not a separate resistor. The base of the transistor 82 isalso connected to a resistor 90. The other end of the resistor 90 isconnected to the voltage source V_(cc). The base of the transistor 82 isfurther connected to one end of a capacitor 92. The other end of thecapacitor 92 is connected through nodes 94, 96 to the emitter of thetransistor 82. A capacitor 98 is connected between node 94 and groundwhile a resistor 100 is connected between node 96 and ground.

While specific values and types of components have been provided below,these are provided only as an example of what those values andcomponents may be. It is not intended to limit the scope of theinvention to the specific values of the components, the manner in whichthe components are connected, or the type of components used. In oneimplementation,the specific values of each of the components of thedifferential dual Colpitts oscillator shown in FIG. 2 are as follows:Transistors 52, 82 may be a bipolar junction transistor such as modelMRF947/MC manufactured by Motorola Corporation. The inductors 54 and 84may have an inductance of 33 nano Henry (nH). The internal resistance58, 88 of the inductors 54, 84 is approximately 3 ohms. The resistors60, 90 may be 25 k ohms and the resistors 70, 100 may be 0.8 k ohms. Thecapacitors 62, 92 may have a capacitance of 33 pico Farads (pF). Thecapacitors 68, 98 may have a capacitance of 22 pF. In this exampleembodiment illustrated in FIG. 2, the voltage supply V_(cc) is 2.7volts.

Certain modifications may be made to the circuit of FIG. 2. For example,the inductors 54 and 84 may be combined into one inductor. The combinedinductor would then only have one internal resistance. DC blockingcapacitors may be added between the inductors and the base of thetransistors in order to prevent the base of the transistors from beingDC grounded.

Advantageously, the differential oscillator of FIG. 2 producesdifferential voltage signals which are out of phase with each other.Because real life inductors 54, 84 have internal resistances 58, 88, thedifferential dual Colpitts oscillator of FIG. 2 oscillates in two modes.It oscillates in its desired fundamental frequency, but it also includesan undesired common mode oscillation. The tendency of node 66 tooscillate in the common mode occurs in the same phase as the tendency ofthe node 96 to oscillate in the common mode. As a result, the circuitenhances instead of suppresses the common mode oscillation problem. As aconsequence, the differential dual oscillator loses energy, with itsmain differential RF output being suppressed during the low voltageswings of the low frequency common mode output.

The common mode oscillation problem that exists in the differential dualColpitts oscillator of FIG. 2 is illustrated in the graph of FIG. 3. Thehorizontal axis depicts the passage of time in microseconds and thevertical axis illustrates the voltage of the differential voltagesignals appearing on nodes 66 and 96. Oscillation in the desiredfundamental high frequency occurs during the portions of the graph 104,108. During portions 104, 108 of the graph, the differential voltagesignals are sinusoidal and 180 degrees out of phase. In this particularexample, the frequency is 300-350 megahertz, but could be roughly 100megahertz to 1 gigahertz.

However, during portions 102, 106 of the graph, the desired highfrequency oscillation disappears and an undesired low frequencyoscillation appears. This particular low frequency oscillation appearsto be roughly 125 kilohertz. The low frequency oscillation is a commonmode oscillation problem because the differential voltage signals are inphase (i.e., move up or down together). As such, the differentialvoltage signals operate in common mode at low frequencies, which causesthe differential voltage to be virtually zero. This common modeoscillation is unpredictable in extent.

FIG. 4 illustrates a second embodiment of the differential dual Colpittsoscillator invention. The differential dual Colpitts oscillator of FIG.4 is comprised of two Colpitts oscillators that have been coupledtogether in a master/slave configuration. This differential oscillatorgenerates differential voltage signals from nodes 138 and 156 where thesignals are out of phase with each other. In one example, thedifferential oscillator is comprised of a bipolar junction transistor122 whose collector is connected to a voltage source V_(cc). The base ofthe transistor 122 is connected through a node 124 to a capacitor 126.The other end of the capacitor 126 is connected through a node 130 to aninductor 128 and to one end of a capacitor 132. The other end of thecapacitor 132 is connected through a node 134 to a capacitor 136 andthrough a node 138 (same as node 134) to the emitter of the transistor122. A resistor 140 is connected between the node 124 and the voltagesource V_(cc) . The capacitor 136 is connected between node 134 andground. A resistor 142 is connected between node 138 and ground.

In one example, the mirror image Colpitts oscillator on the right handside of FIG. 4 is comprised of a transistor 144 which is substantiallyidentical to the transistor 122. The collector of the transistor 144 isalso connected to the voltage source V_(cc). The base of the transistor144 is connected through a node 146 to a capacitor 148 and a resistor160. The other end of the capacitor 148 is connected through a node 150to an inductor 152 and one end of a capacitor 154. The other end of thecapacitor 154 is connected through a node 156 to a capacitor 158, to theemitter of the transistor 144 and to a resistor 162. The remaining endsof the inductor 152, the capacitor 158 and the resistor 162 areconnected to ground.

The values of the components in FIG. 4 may be the same as the values inFIG. 2. Capacitors 126 and 148 are DC blocking capacitors which preventthe base of the transistors 122 and 144 from being DC grounded. Thesetransistors require bias to operate and thus, DC grounding is to beavoided. In the alternative, the capacitors 126 and 148 may be moved tothe inductor side of the nodes 130, 150 if the capacitance values arelarge enough. For instance, the capacitors 126, 148 may have acapacitance of 220 pF to 2200 pF.

Effectively, the pair of inductors 128, 152 forms a cross-coupledtransformer. In one example, this circuit produces differential voltagesignals at nodes 138 and 156 that are precisely 180 degrees out ofphase, as desired. This circuit is able to produce such differentialsignals even with some stray capacitance 166 across the transformerbecause of the symmetry of the circuit. As can be seen, there is a planeof symmetry between inductors 128 and 152 in FIG. 4. This circuiteliminates the low frequency common mode oscillation problem since thetransformer with its opposite-phase windings forces all coupling to beout of phase. Common mode coupling is only possible through the straycapacitance. However, if this capacitance becomes large, the circuitwill not oscillate at all because the capacitance effectively shorts outthe inductance of the transformer.e

Therefore, this embodiment of a differential dual Colpitts oscillatorcreates differential voltage signals, but may not always be optimalbecause of the practical undesirability of wirewound transformers.

FIG. 5 illustrates a third embodiment of the differential oscillatorinvention. Specifically, FIG. 5 depicts a differential dual Colpittsoscillator 180. In one example, the differential dual Colpittsoscillator 180 essentially comprises two mirror image Colpittsoscillators, with some differences which will become apparent. Thetransistor 182 may be a bipolar junction transistor. In one example, thetransistor 182 has its collector connected to a voltage source V_(cc).The base of the transistor 182 is connected to an inductor 184 and oneend of a capacitor 186. The other end of the capacitor 186 is connectedthrough a node 187 to the emitter of the transistor 182, to one end of acapacitor 188 and to a resistor 190. The resistor 190 and the capacitor188 have their other ends connected to ground.

The transistor 192 also has its collector connected to the voltagesource V_(cc). The base of the transistor 192 is connected to the otherend of the inductor 184 and to one end of a capacitor 194. The other endof the capacitor 194 is connected through a node 196 to the emitter ofthe transistor 192, one end of a capacitor 198 and to one end of aresistor 200. The resistor 200 and the capacitor 198 have their otherends connected to ground. Finally, a resistor 202 is connected betweenthe inductor 184 and the voltage source V_(cc). The resistor 202 isconnected to the middle of the inductor 184.

It should be noted that the transistors in the differential oscillatorsdescribed in this patent application may be bipolar junctiontransistors, mosfet transistors, jfet transistors, or any other kind oftransistors. Since the differential oscillators described in thisapplication are based on mirror image oscillators, in one example, thetransistors of the oscillators are advantageously the same type oftransistor. In other words, if one transistor is a bipolar junctiontransistor, the transistor in the mirror image oscillator isadvantageously also a bipolar junction transistor. Further, thetransistors in the two mirror image oscillators in this example areadvantageously matched with respect to all their characteristics inorder to achieve an accurate 180 degree phase shift.

The values of the components in FIG. 5 may be varied to suit theparticular needs of the application. For example, the values could be asfollows: the inductor 184 has an inductance of 66 nH, the resistors 190and 200 have a resistance of 800 ohms, the resistor 202 has a resistanceof 30 ohms, the capacitors 186 and 194 have a capacitance of 33 pF andthe capacitors 188 and 198 have a capacitance of 22 pF.

Because the differential dual Colpitts oscillator 180 of FIG. 5eliminates the problems associated with the alternative embodiments asshown in FIGS. 2 and 4, it may be preferred in some circumstances. Likethe alternative embodiments of FIGS. 2 and 4, in one example, thedifferential dual Colpitts oscillator 180 of FIG. 5 is able to outputdifferential voltage signals from the nodes 187 and 196 respectivelythat are precisely 180° out of phase from each other. Resistor 202 actsas a very low impedance element. However, the very low impedanceintroduced by resistor 202 at the plane of symmetry of the circuit stopsthe low frequency common mode of oscillation. It does so withoutstopping the desired high frequency differential mode of oscillationbecause the balanced signal exhibits a voltage amplitude of zero at thispoint. In fact, the resistor 202 could be reduced to zero ohms and thedesired high frequency oscillation would still occur, except that thetwo mirror image Colpitts oscillators would then be completely uncoupledand no longer forced to be 180 degrees out of phase. With a small butnon-zero value of the resistor 202, a small amount of coupling betweenthe oscillators forces them to operate in lock-step with a 180 degreephase difference.

The value of the resistor 202 is selected so that the desireddifferential gain is obtained while also the common mode gain iseliminated. If the value of the resistor 202 is too small, it couldundesirably eliminate the desired coupling between the two Colpittsoscillators, effectively acting as a short to AC ground and reducing thecircuitry into two independent and separate Colpitts oscillators. on theother hand, if the resistance of the resistor 202 is too large, thecircuit might not eliminate enough of the common mode gain, resulting insome undesired common mode oscillation.

Depending on the desired circuit characteristics, the resistor 202 maybe selected to be, for example, between 25 and 300 ohms. For low powerdevices running in the milliwatt range, the resistor 202 can be 30 to 50ohms for example.

The differential oscillator of FIG. 5 has many advantages. Thedifferential oscillator is simple to implement, has a low outputimpedance, needs only a small amount of current, creates differentialoutput voltage signals that are about 180 degrees out of phase with eachother and have excellent common mode rejection capability. Further, thedifferential oscillator uses Colpitts oscillators whose individualoperating characteristics are well known. For example, the amount ofphase shift, noise and other operating characteristics of a Colpittsoscillator are known.

FIG. 6 is a schematic that illustrates a fourth embodiment of thedifferential oscillator. It is virtually identical to the differentialdual Colpitts oscillator of FIG. 5, but replaces the single inductor 184with two inductors: a first inductor 183 and a second inductor 185. Thefirst inductor 183 and the second inductor 185 are separated by the node210 to which the resistor 202 is connected. Rather than connect theresistor 202 to the middle of the inductor 184, the inductor 184 hasbeen split into two equal inductors 183 and 185 and the resistor 202 isconnected to the node in between the inductors. This approach simplifiesthe task of connected the resistor 202 to the middle of the inductor184. The resistors 206 and 208 represent the internal resistances of thefirst inductor component 183 and the second inductor component 185respectively.

While the values of the components of the differential oscillator ofFIG. 6 may be changed to suit the particular needs of the application,the values may include those of FIG. 5. Each of the first inductor 183and second inductor 185 may have an inductance of 33 nH and theirinternal resistances 206 and 208 may be 3 ohms each.

At times, the circuit designer may want to use an oscillator whosefrequency of oscillation is adjustable. For example, telecommunicationsystems often require the selection of a channel from among manychannels because a given transceiver is assigned different carrierfrequencies at different times. Thus, the resonant frequency must beadjustable by well-defined steps. If the frequency of oscillation can bevaried by a voltage, the circuit is called a voltage controlledoscillator (VCO).

FIG. 10 is a time response of the differential oscillator of FIG. 6(which essentially is the differential oscillator of FIG. 5). Thehorizontal axis depicts the passage of time in microseconds (from zeroto 1.6 microseconds) and the vertical axis illustrates the voltage ofthe differential voltage signals appearing on nodes 187 and 196. As canbe seen, oscillation of the differential voltage signals occurs 180degrees out of phase without the common mode oscillation problem whichappears in FIG. 3. FIG. 11 is the time response graph of FIG. 10 but ata different time scale. The horizontal time axis is spread out from 2nanoseconds to 200 nanoseconds to show that the differential voltagesignals 170, 172 occur 180 degrees out of phase. FIG. 12 is another timeresponse graph that shows the graph of FIG. 10 at yet another timescale, spanning from 200 nanoseconds to 220 nanoseconds. Again, FIGS. 11and 12 show that the oscillation of the differential voltage signalsoccurs 180 degrees out of phase without the common mode oscillationproblem which appears in FIG. 3.

FIG. 7 is a schematic of a first embodiment of a voltage controlledoscillator that is based on the circuit of FIGS. 5. FIG. 7 includes avaractor or variable capacitor 204. Variable capacitor 204 allows one tochange the inductance of the inductor 184, and thus the LC tank, so thatthe resonant frequency of the oscillator can be varied. As a result,through the addition of the variable capacitor 204, the differentialdual Colpitts oscillator of FIGS. 5 now behaves as a voltage controlledoscillator (VCO). Alternatively, if the inductor 184 were replaced bytwo separate inductors, a varactor could be placed in parallel over eachof the inductors. Similarly, FIG. 7 could be based on any otherembodiment of the differential oscillator previously discussed.

FIG. 8 is a second embodiment of a voltage controlled oscillator that isbased on the circuit of FIG. 6 where same components have been given thesame reference numerals. Notably, FIG. 8 could also be based on anyother embodiment of the differential oscillator previously discussed. Ascan be seen, FIG. 8 has two inductors 183, 185, which could be replacedby a single inductor. FIG. 8 includes two variable capacitors 260, 262;whereas, one example of FIG. 7 only had one variable capacitor. One endof the first variable capacitor 260 is connected to a control signalV_(ctrl) and to one end of the second variable capacitor 262. The otherend of the first variable capacitor 260 is connected to a resistor 264and a capacitor 266. The other end of the resistor 264 is connected toground, while the other end of the capacitor 266 is connected to thebase of the transistor 182. Likewise, one end of the second variablecapacitor 262 is connected to the control signal V_(ctrl) and to thefirst variable capacitor 260. The other end of the second variablecapacitor 262 is connected to a resistor 268 and to a capacitor 270. Theother end of the resistor 268 is connected to ground, while the otherend of the capacitor 270 is connected to the base of the transistor 192.

The capacitors 266 and 270 act as DC blocking capacitors to blockV_(ctrl) from having undesired DC paths. The only desired DC path forV_(ctrl) is through resistors 264, 268 to ground. The resistors 264 and268 completes the DC loop and allows V_(ctrl) to be applied across thevariable capacitors 260, 262.

Although the values of the components in FIG. 8 may be changed to suitthe particular needs of the application, one set of examples values mayinclude the following. The values of like components as compared to FIG.6 may have like values. For example, the resistors 264 and 268 may havevalues in the range of 2k to 5k ohms and the capacitors 266 and 270 mayhave values in the range of 1 to 100 pF. If the capacitors 266, 270 aremade smaller, the varactors 260, 262 have a smaller effect on thefrequency of oscillation. By contrast, if the capacitance of capacitors266, 270 is increased, the effect of the varactors 260, 262 on thefrequency of oscillation is increased.

In operation, if one increases the DC voltage applied to V_(ctrl), thecapacitance of the varactors 260 and 262 decreases, the capacitanceacross the inductors 183 and 185 decreases, the total impedance acrossthe LC tank decreases, the effective inductance of the circuitdecreases, and the frequency of oscillation increases. By contrast, ifone decreases the DC voltage applied to V_(ctrl), the capacitance of thevaractors 260 and 262 increases, the capacitance across the inductors183 and 185 increases, the total impedance across the LC tank increases,the effective inductance of the circuit increases, and the frequency ofoscillation decreases.

Alternatively, a first variable capacitor could be placed in parallelover the first inductor 183 and a second variable capacitor placed inparallel over the second inductor 185.

FIG. 9 illustrates a block diagram of a transceiver that uses thedifferential oscillator. In fact, this particular transceiver uses twodifferential dual Colpitts oscillators 300 and 318. The transceiver isparticularly suited for a telecommunications device such as a cellularphone of virtually any type. If the cellular phone is a traditionalcellular phone, e.g., based on the American Mobile Phone System (AMPS),both the transmit and receive circuits operate at the same time, thoughat different frequencies. If the cellular phone is based on GlobalSystem for Mobile communications (GSM) or Time Division Multiple Access(TDMA), only one of the transmit or receive circuits can be on at atime. Certainly, the cellular phone using the differential oscillatorcan be based on other systems and variants such as Digital CellularSystem at 1800 MHz (DCS-1800) which is GSM operating at 1800 MHz insteadof 900 MHz, Interim Standard 136 (IS-136) which is a TDMA system,Code-Division Multiple Access (CDMA or IS-95), or PersonalCommunications Services (PCS).

Advantageously, this transceiver uses differential signals to achievegreater immunity from external noise and interference. Because externalinterference is likely to affect internal signals equally or close toequally, the use of a differential signal effectively cancels theexternal interference. For this reason, the transceiver 298 usesdifferential signals.

A transmit baseband is an intelligible signal to be sent outside thecellular phone and may include voice or data information. This transmitbaseband is sent to a mixer 302 that modulates the baseband signal ontoa carrier signal. Actually, the mixer 302 may be any device thatmodulates the baseband signal onto a carrier signal including, forexample, a FM modulator, in-phase and quadrature (IQ) modulator andupconverter. The carrier signal is a very high frequency (VHF)differential signal produced by the differential VHF oscillator 300. Thedifferential VHF oscillator 300 may be any of the differentialoscillators described herein and preferably is the differential dualColpitts oscillator of FIG. 6. VHF ranges roughly from 100 MHz to 400MHz.

The output of the mixer 302 is a carrier VHF signal modulated with thebaseband signal that is received by a passband filter 304. The passbandfilter 304 only allows certain frequencies to pass through and outputs a“transmit IF” signal. The transmit IF signal is an intermediatefrequency signal which is received by another mixer 306. The mixer 306converts the transmit IF signal into a radio frequency (RF) signal to betransmitted. Mixers create two types of signals: additive signals andsubtractive signals. In other words, when a mixer mixes two incomingsignals, the mixer outputs a signal whose frequency is the sum of thefrequencies of the two incoming signals as well as a second signal whosefrequency is the difference of the frequencies of the two incomingsignals. Thus, the mixer results in some undesired frequencies. Themixer 306 modulates the filtered signal from the filter 304 onto thedifferential ultra high frequency (UHF) signal generated by thedifferential UHF oscillator 318. UHF ranges roughly from 700 MHz to 2GHz.

The passband filter 308 receives the modulated signal from the mixer 306and passes only the desired frequencies to a power amplifier 310. Thepower amplifier transmits the desired frequencies over an antenna (notshown).

At the receiving end of the transceiver 298, signals of varyingfrequencies are received by the antenna and passed to a low noiseamplifier 312. The low noise amplifier 312 amplifies the signals andsends them to a passband filter 314. The passband filter 312 selects adesired frequency or frequencies out of the plurality of incomingfrequencies where the desired frequency is expected to carry the voiceor data information of interest. For example, the passband filter 312may select 900 megahertz signals.

The mixer 316 receives the filtered signal from the passband filter 312and the differential UHF signal from the differential UHF oscillator318. The mixer 316 uses these signals to demodulate the filteredsignals. For example, it converts the 900 MHz signal into a 100 MHzsignal. The differential UHF oscillator 318 helps by acting as a tuningoscillator. The oscillator 318 is a differential oscillator of the typedescribed in this patent application and preferably is the differentialdual Colpitts oscillator of FIG. 6. The differential UHF oscillator 318is tuned to generate a specific desired frequency. In using the exampleprovided above, the oscillator 318 may be tuned to generate a 800 MHzsignal. As such, the mixer 316 receives the 900 MHz signal from thepassband filter 314 and a 800 MHz signal from the differential UHFoscillator 318. The mixer 316 outputs a subtractive signal of 100 MHzand an additive signal of 1700 MHz. These two signals are passed to apassband filter 320 which selects the desired frequency, for example,100 MHz here.

This approach allows the system to readily use multiple channels forcommunication. For example, if one wants to change the channel, onesimply changes the frequency output from the differential UHF oscillator318 instead of changing the passband filter 314 to select a differentfrequency. This arrangement permits the system to use fixed passbandfilters and yet have multiple channels. For example, if the channel ischanged, the passband filter 314 need not be changed to pass 901 MHzinstead of 900 MHz. Rather, the differential UHF oscillator 318 is tunedto 801 MHz instead of 800 MHz. The mixer 316 would then take the 901 MHzsignal and 801 MHz signal and output a desired 100 MHz signal as thesubtractive signal and an extraneous 1702 MHz signal as the additivesignal. Therefore, in one example the intermediate frequency is fixed at100 MHz regardless of the channel the system is tuned to.

The passband filter 320 receives the signal from the mixer 316 and, inthis example, would pass the desired 100 MHz signal and block theundesired 1702 MHz signal.

The “receive IF” signal is an intermediate frequency signal that isreceived by the mixer 322. This signal contains the voice or data signalmodulated onto the carrier signal. Voice frequencies usually fallbetween 200 Hz and 3 kHz. Data frequencies can go higher, up toarbitrarily high frequencies. For example in GSM, the channel bandwidthis currently 200 kHz.

The differential VHF oscillator 300 is tuned to the carrier frequency(e.g., 100 MHz in the first example). The mixer 322 receives thedifferential VHF signal and the output of the filter 320. The mixer 322outputs a desired subtractive signal that contains the voice or datasignal only as well as an undesired additive signal.

The low pass filter 324 receives the signals from the mixer 322,eliminates the undesired additive signal and allows the desiredsubtractive signal (e.g., the voice or data signal) to pass through asthe receive baseband.

While particular embodiments, implementations, and implementationexamples of the present invention have been described above, it shouldbe understood that they have been presented by way of example only, andnot as limitations. The breadth and scope of the present invention isdefined by the following claims and their equivalents, and is notlimited by the particular embodiments described herein.

What is claimed is:
 1. A differential oscillator comprising: (a) aninductive element; (b) a first resistive element coupled to saidinductive element and to a voltage source, wherein said first resistiveelement is coupled to said inductive element at a point between saidfirst and second ends of said inductive element; (c) a first oscillatorcoupled to a first end of said inductive element and generating a firstoscillating voltage signal; (d) a second oscillator which generallycorrelates with said first oscillator, said second oscillator beingcoupled to a second end of said inductive element and generating asecond oscillating voltage signal being out of phase with said firstoscillating voltage signal.
 2. The differential oscillator of claim 1wherein said first oscillating voltage signal is about 180 degrees outof phase with said second oscillating voltage signal.
 3. Thedifferential oscillator of claim 1 wherein said first oscillator is afirst Colpitts oscillator and said second oscillator is a secondColpitts oscillator that is a mirror image of said first Colpittsoscillator.
 4. The differential oscillator of claim 1 wherein said firstoscillator further comprises: a load resistance element coupled toground; a first transistor including a first terminal, a second terminaland a third terminal; said first terminal is coupled to said voltagesource; said second terminal is coupled to said first end of saidinductive element; and said third terminal outputs said firstoscillating voltage signal and is coupled to said load resistanceelement.
 5. The differential oscillator of claim 4 wherein said secondoscillator further comprises: a load resistance element coupled toground; a first transistor including a first terminal, a second terminaland a third terminal; said first terminal is coupled to said voltagesource; said second terminal is coupled to said second end of saidinductive element; and said third terminal outputs said secondoscillating voltage signal and is coupled to said load resistance. 6.The differential oscillator of claim 4 wherein said first oscillatorfurther comprises: (a) a first capacitor coupled between said secondterminal of said first transistor and said third terminal of said firsttransistor; (b) a second capacitor coupled between said third terminalof said first transistor and ground; and (c) a second resistive elementcoupled between said third terminal of said first transistor and ground.7. The differential oscillator of claim 6 wherein said mirror imageoscillator further comprises: (a) a third capacitor coupled between saidsecond terminal of said second transistor and said third terminal ofsaid second transistor; (b) a fourth capacitor coupled between saidthird terminal of said second transistor and ground; and (c) a thirdresistive element coupled between said third terminal of said secondtransistor and ground.
 8. The differential oscillator of claim 4 whereinsaid first transistor is a bipolar junction transistor; said firstterminal of said first transistor is the collector of said firsttransistor; said second terminal of said first transistor is the base ofsaid first transistor; and said third terminal of said first transistoris the emitter of said first transistor.
 9. The differential oscillatorof claim 8 wherein said second transistor is a bipolar junctiontransistor; said first terminal of said second transistor is thecollector of said second transistor; said second terminal of said secondtransistor is the base of said second transistor; and said thirdterminal of said second transistor is the emitter of said secondtransistor.
 10. The differential oscillator of claim 3 wherein saidfirst Colpitts oscillator further comprises: (a) a first capacitorcoupled between said second terminal of said first transistor and saidthird terminal of said first transistor; (b) a second capacitor coupledbetween said third terminal of said first transistor and ground; and (c)a second resistive element coupled between said third terminal of saidfirst transistor and ground; and said second Colpitts oscillator furthercomprises: (a) a third capacitor coupled between said second terminal ofsaid second transistor and said third terminal of said secondtransistor; (b) a fourth capacitor coupled between said third terminalof said second transistor and ground; and (c) a third resistive elementcoupled between said third terminal of said second transistor andground.
 11. The differential oscillator of claim 5 further comprising: afirst DC blocking capacitor coupled to said second terminal of saidfirst transistor and to said inductive element; a second DC blockingcapacitor coupled to said second terminal of said second transistor andto said inductive element.
 12. A differential oscillator comprising: (a)an inductive element; (b) a first resistive element coupled to saidinductive element and to a voltage source; (c) a first oscillatorcoupled to a first end of said inductive element and generating a firstoscillating voltage signal; (d) a second oscillator which generallycorrelates with said first oscillator, said second oscillator beingcoupled to a second end of said inductive element and generating asecond oscillating voltage signal being out of phase with said firstoscillating voltage signal; and (e) wherein said inductive elementcomprises a first inductor and a second inductor; said first inductor isconnected to said second inductor at a node; said first resistiveelement is connected to said node; said first oscillator is coupled tosaid first inductor; and said mirror image oscillator is coupled to saidsecond inductor.
 13. A differential oscillator comprising: a firstresistive element coupled to ground; a second resistive element coupledto ground; a first transistor having a first terminal, a second terminaland a third terminal, said first terminal being coupled to a voltagesource and said third terminal being coupled to said first resistiveelement; a second transistor being substantially identical to said firsttransistor, said second transistor having a fourth terminal, a fifthterminal and a sixth terminal, said fourth terminal being coupled tosaid voltage source and said sixth terminal being coupled to said secondresistive element; an inductive element having a first end coupled tosaid second terminal of said first transistor and a second end coupledto said fifth terminal of said second transistor; said first resistiveelement coupled to said inductive element and to said voltage source,wherein said first resistive element is coupled to said inductiveelement at a point between said first and second ends of said inductiveelement.
 14. The differential oscillator of claim 13 further comprising:a first capacitor coupled to said second terminal and said thirdterminal of said first transistor; a second capacitor coupled betweensaid third terminal of said first transistor and connected to ground; athird capacitor coupled to said second terminal and said third terminalof said second transistor; a fourth capacitor coupled between said thirdterminal of said second transistor and connected to ground; a secondresistive element coupled to said third terminal of said firsttransistor and connected to ground; and a third resistive elementcoupled to said third terminal of said second transistor and connectedto ground.
 15. A differential oscillator comprising: a first resistiveelement coupled to ground; a second resistive element coupled to ground;a first transistor having a first terminal, a second terminal and athird terminal, said first terminal being coupled to a voltage sourceand said third terminal being coupled to said first resistive element; asecond transistor being substantially identical to said firsttransistor, said second transistor having a fourth terminal, a fifthterminal and a sixth terminal, said fourth terminal being coupled tosaid voltage source and said sixth terminal being coupled to said secondresistive element; an inductive element having a first end coupled tosaid second terminal of said first transistor and a second end coupledto said fifth terminal of said second transistor; whereby said inductiveelement further comprises a first inductor connected to a secondinductor at a node; and said first resistive element is connected tosaid node and is coupled to said voltage source; whereby the voltage thevoltage signal on said third terminal of said first transistoroscillates at a phase which is about 180 degrees out of phase with thevoltage signal on said sixth terminal of said second transistor.
 16. Adifferential oscillator comprising: (a) an inductive element; (b) afirst Colpitts oscillator circuit including a first resistive elementcoupled to ground, wherein said first resistive element is coupled tosaid inductive element at a point between said first and second ends ofsaid inductive element; a first transistor having a first terminal, asecond terminal and a third terminal; said first terminal is coupled toa voltage source; said second terminal is coupled to said inductiveelement; said third terminal is coupled to said first resistive elementand carrying a first oscillating voltage signal; (c) a second Colpittsoscillator which generally correlates with said first Colpittsoscillator, said second Colpitts oscillator having a second resistiveelement coupled to ground; a second transistor having a fourth terminal,a fifth terminal and a sixth terminal; said fourth terminal is coupledto a voltage source; said fifth terminal is coupled to said inductiveelement; said sixth terminal is coupled to said second resistive elementand carrying a second oscillating voltage signal, where said firstoscillating voltage signal is about 180 degrees out of phase with saidsecond oscillating voltage signal.
 17. A differential oscillatorcomprising: a first resistive element coupled to ground; a firsttransistor having a first terminal, a second terminal and a thirdterminal, said first terminal is coupled to a voltage source, said thirdterminal is coupled to said first resistive element; a first inductorhaving a first end coupled to said second terminal of said firsttransistor; a second resistive element coupled to ground; a secondtransistor substantially identical to said first transistor, said secondtransistor having a first terminal, a second terminal and a thirdterminal, said first terminal is coupled to a voltage source, said thirdterminal is coupled to said second resistive element; a second inductorhaving a first end coupled to said second terminal of said secondtransistor; said first resistive element connected to said secondterminal of said first transistor and coupled to said voltage source;said second resistive element connected to said second terminal of saidsecond transistor and coupled to said voltage source; said thirdterminal of said first transistor outputs a first oscillating voltagesignal, said third terminal of said second transistor outputs a secondoscillating voltage signal that oscillates at a phase about 180 degreesout of phase with said first oscillating voltage signal; wherein saidfirst inductor is connected to said second inductor at a node; saidfirst resistive element is connected to said node; said first oscillatoris coupled to said first inductor; and said mirror image oscillator iscoupled to said second inductor.
 18. The differential oscillator ofclaim 17 wherein said first inductor has a second end connected toground and said second inductor has a second end connected to ground.19. The differential oscillator of claim 17 wherein said first inductorhas a second end coupled to the second end of said second inductor. 20.The differential oscillator of claim 17 further comprising: (a) a firstcapacitor coupled between said second terminal of said first transistorand said third terminal of said first transistor; (b) a second capacitorcoupled to said third terminal of said first transistor and connected toground; (c) a third capacitor coupled between said second terminal ofsaid second transistor and said third terminal of said secondtransistor; (d) a fourth capacitor coupled to said third terminal ofsaid second transistor and connected to ground; (e) a third resistiveelement coupled to said third terminal of said first transistor andconnected to ground; and (f) a fourth resistive element coupled to saidthird terminal of said second transistor and connected to ground.
 21. Avoltage controlled oscillator comprising; (a) an inductive element; (b)a first resistive element coupled to said inductive element and to avoltage source; (c) a first oscillator coupled to one end of saidinductive element, said first oscillator outputting a first oscillatingvoltage signal; (d) a second oscillator which generally correlates withsaid first oscillator, said second oscillator being coupled to the otherend of said inductive element and outputting a second oscillatingvoltage signal that is about 180 degrees out of phase with said firstoscillating voltage signal; (e) a variable capacitance element coupledacross said inductive element, where varying the capacitance of saidvariable capacitance element changes the frequency of oscillation ofsaid first and second oscillating voltage signals; (f) wherein saidinductive element comprises a first inductor and a second inductor; saidfirst inductor is connected to said second inductor at a node; saidfirst resistive element is connected to said node; said first oscillatoris coupled to said first inductor; and said mirror image oscillator iscoupled to said second inductor.
 22. The voltage controlled oscillatorof claim 21 wherein said variable capacitance element is a varactor. 23.The voltage controlled oscillator of claim 21 wherein said variablecapacitance element is a variable capacitor.
 24. The voltage controlledoscillator of claim 21 wherein said variable capacitance element iscoupled across said first inductor.
 25. The voltage controlledoscillator of claim 21 wherein said variable capacitance element iscoupled across both said first inductor and said second inductor. 26.The voltage controlled oscillator of claim 21 wherein said variablecapacitance element comprises a first variable capacitance element and asecond variable capacitance element; said first variable capacitanceelement is coupled across said first inductor and not coupled acrosssaid second inductor; and said second variable capacitance element iscoupled across said second inductor and not coupled across said firstinductor.
 27. The voltage controlled oscillator of claim 21 wherein saidvariable capacitance element comprises a first variable capacitanceelement and a second variable capacitance element; said first variablecapacitance element and said second variable capacitance element arecoupled in series across both said first inductor and said secondinductor.
 28. The voltage controlled oscillator of claim 21 wherein saidfirst oscillator further comprises (a) a first resistive element coupledto ground; (b) a first transistor having a first terminal, a secondterminal and a third terminal; (c) said first terminal is coupled tosaid voltage source; (d) said second terminal is coupled to another endof said inductive element; (e) said third terminal outputs said firstoscillating voltage signal and is coupled to said first resistiveelement; said second oscillator further comprises (a) a second resistiveelement coupled to ground; (b) a second transistor that is substaniallyidentical to said first transistor, said second transistor having afourth terminal, a fifth terminal and a sixth terminal; (c) said fourthterminal is coupled to said voltage source; (d) said fifth terminal iscoupled to another end of said inductive element; (e) said sixthterminal is coupled to said second resistive element and outputs saidsecond oscillating voltage signal, said second oscillating voltagesignal being about 180 degrees out of phase with said first oscillatingvoltage signal.
 29. The voltage controlled oscillator of claim 28further comprising: (a) a first capacitor coupled between said secondterminal of said first transistor and said third terminal of said firsttransistor; (b) a second capacitor coupled between said third terminalof said first transistor and connected to ground; (c) a third capacitorcoupled between said fifth terminal of said second transistor and saidsixth terminal of said second transistor; (d) a fourth capacitor coupledbetween said sixth terminal of said second transistor and connected toground; and (e) a second resistive element coupled between said thirdterminal of said first transistor and connected to ground; (f) a thirdresistive element coupled between said sixth terminal of said secondtransistor and connected to ground.
 30. The voltage controlledoscillator of claim 28 further comprising: a first DC blocking capacitorcoupled to said second terminal of said first transistor and saidinductive element; a second DC blocking capacitor coupled to said fifthterminal of said second transistor and said inductive element.
 31. Thevoltage controlled oscillator of claim 27 further comprising: a firstcapacitor coupled between said first variable capacitance element andsaid first inductor; a second capacitor coupled between said secondvariable capacitance element and said second inductor.
 32. Adifferential oscillator comprising: (a) a low impedance elementcomprising a resistor; (b) a first oscillator including a firstresistance element coupled to ground; a first capacitance element; asecond capacitance element coupled to ground; a first inductive element;a first transistor including a first terminal, a second terminal and athird terminal; said first terminal is coupled to a voltage source; saidsecond terminal is coupled to said first inductive element and to saidfirst capacitance element; and said third terminal is coupled to saidfirst resistive element, said first capacitance element and said secondcapacitance element; (c) a second oscillator that is a mirror image ofsaid first oscillator and coupled to said first oscillator at a plane ofsymmetry, said second oscillator including a second resistance elementcoupled to ground; a third capacitance element; a fourth capacitanceelement coupled to ground; a second inductive element; a secondtransistor including a fourth terminal, a fifth terminal and a sixthterminal; said fourth terminal is coupled to said voltage source; saidfifth terminal is coupled to said second inductive element and to saidthird capacitance element; and said sixth terminal is coupled to saidsecond resistive element, said third capacitance element and said fourthcapacitance element; wherein said low impedance element is located atsaid plane of symmetry between said first oscillator and said secondoscillator.
 33. The differential oscillator of claim 32 wherein said lowimpedance element is shared between said first oscillator and saidsecond oscillator.
 34. The differential oscillator of claim 32 whereinsaid first inductive element and said second inductive element form asingle inductor such that said first oscillator and said secondoscillator share said single inductor.
 35. A differential oscillatorcomprising: first and second single-ended output oscillators whichgenerally correlate with one another and which are coupled through acoupling network, wherein the coupling network comprises a cross-coupledtransformer; and a differential output formed from the single-endedoutputs of the first and second oscillators.
 36. The differentialoscillator of claim 35 wherein the first and second oscillators areColpitts oscillators.
 37. The differential oscillator of claim 35wherein the coupling network comprises inductors from each of the firstand second oscillators.
 38. The differential oscillator of claim 35wherein the coupling network comprises an inductor and a resistiveelement coupled at one end to a voltage source and at the other end tothe inductor.
 39. The differential oscillator of claim 38 wherein theresistance of the resistive element is such as to eliminate or reducecommon mode oscillation.
 40. A differential oscillator comprising: firstand second single-ended output oscillators which generally correlatewith one another and which are coupled through a coupling network,wherein the coupling network is configured to reduce or eliminate commonmode oscillation; and a differential output formed from the single-endedoutputs of the first and second oscillators.
 41. A differentialoscillator comprising: first and second single-ended output oscillatorswhich generally correlate with one another and which are coupled througha coupling network, wherein the coupling network comprises first andsecond inductors coupled at a node, and a resistive element coupled atone end to a power supply and at the other end to the node; and adifferential output formed from the single-ended outputs of the firstand second oscillators.
 42. The differential oscillator of claim 41wherein the resistance of the resistive element is such as to eliminateor reduce common mode oscillation.